The use of wadded conductor contacts or "buttons" mounted in insulator substrates to form "button boards" is a known type of interface device for electronic circuit coupling. They typically provide both direct coupling and physical separation between electronic circuits, which are commonly formed on adjacent circuit boards. Most frequently, resilient bundles or "wads" of fine electric current conductors are retentively engaged in corresponding holes in or passing through the non-conductive substrate carrier board. The ends of these wads or "buttons" are exposed and typically protrude at the respective surface of the insulative carrier board; see for example U.S. Pat. No. 4,581,679 and 4,574,331. Such conductive wads have very low resistance to current when their exposed ends or "buttons" are compressively engaged with surface contact pad areas on the circuit boards.
Furthermore, because their ratio of diameter to length in a compressed state is considerably larger than contacts previously known in the connector art and because of their random internal multi-contact composition, such wadded conductor elements have relatively low capacitance and inductance, and so they provide relatively low impedance for dynamic electronic circuit configurations, such as are used for high speed data processing and other high bandwidth applications.
Even though such button boards are technically superior to many other electronic circuit interconnection arrangements, previously proposed designs have presented a number of practical problems in their fabrication and use. Heretofore, cylindrical button contacts of wadded fine conductor wires have been inserted axially into generally uniform cylindrical holes which were formed in the substrate such as by acid etching of ceramicized glass substrates or drilling a laminated or sheet plastic insulator sheet. The button wads fill the respective holes and are held in place in their corresponding holes by compressive radial frictional engagement with the side walls of each of the holes. Because of this relationship, insertion of the button wads into their respective holes has been a difficult process. The threshhold problem was in feeding or threading the leading end of each wad into the respective hole. Further, as each wad is so inserted, insertion resistance increases with increasing insertion depth because the wad-to-hole wall contact area increases with increasing insertion depth. This insertion relationship also made the simple wad-filled hole construction unsuitable for use of long button contacts through substrates having significant thickness, because of the great insertion resistance.
Although the restricted diameter of the holes was deemed necessary to satisfactorily retain the inserted wads, the resulting frictional engagement of the wads with the holes impaired the spring movement of the contacts and hence reduced the effective desirable resiliency of the inserted wads. This was especially true when the holes were formed by etching or drilling, because any roughness or surface discontinuities on the hole walls increased the friction and/or catching of the fine conductors of the contact wads on the walls of the holes. The impairment of spring action movement of the contacts could adversely effect the positioning of the contact ends and cause variances in the compressive engagement of the multiple conductive strand elements making up the contact end surface with an opposed conductive contact surface, with attendant unpredictability of the electrical resistance through the resulting contact interface.
Further, any strand segment or segments of the contact which were misaligned with the respective hole, either because of spreading or "mushrooming" of the protruding contact end or any pulling or other lateral detachment or displacement of a strand segment from the cylindrical contact body became "loose strands" which could be caught between the substrate and the adjacent mating components. This would preclude proper surface-to-surface seating of the component on the substrate and correspondingly limit the compressive force on the main body of the contact and also effect the resultant electrical resistance through the contacts. Such loose strands also can cause short circuits to adjacent conductors on the interconnect substrate or on the respective mating component, such as a circuit board.
The lack of free movement of the contact ends can also cause the buttons to shift off center when compressed in use.
Of course, if the hole diameter is increased to permit a greater degree of resiliency for the buttons, the wad will not be as securely retained in the hole, and the. buttons can be easily dislodged during handling and, in some instances, during use.